Nice concept
I like the story concept; it's quite unlike any other stories I've heard of before. But, there are a few parts you could polish up on this.
-The art style is nice, but your anatomy could use some work-- namely hands, profile views, and proportions. I noticed the hands looked somewhat rushed. If this was because you didn't want to draw the same hands over and over, that's understandable, but it's advisable that you take some time drawing them anyway, because it will help make your flash look better. As far as profiles go, the jaw should be more defined and convex-- they tend to be blockier in males, and the way you drew the green haired boy's jaw makes it look like it's concave. The ear also looks kind of squished, and the neck also looks a little too skinny. The only big thing I think I should point out about proportions, though, is that the green haired boy's arm was far too short. His elbow should be approximately level with his navel, and his hands should come down to about the middle of his thighs. Of course, this is all sort of nit-picky, so if you didn't want to focus on these aspects, that's understandable. I just think you could improve the animation quality by focusing a little on some good solid drawing. :)
-Speaking of animation, the motion you have here is kind of limited. It's pretty choppy, and it seems like you relied quite a bit on pans of still images and stills with a little bit of movement (like the teardrop). I also thought the green haired boy's head turns a bit too quickly to match his mood. Try slowing it down a bit, and draw more in-between frames to make it run smoother. The part when the angel reaches her hand up could also use some work-- her body lost volume when she moved. This makes for some passable yet unconvincing artwork. If you pay close attention to how big something is when you're doing frame-by-frame animation, the overall quality will get better dramatically. :) In the part where the white-haired boy swung the sword, the hand placement on the sword looked kind of awkward, not to mention the swinging of the sword went by too quickly to properly register what happened. Also, when he gets those shocked-looking eyes, I think it would help a lot to put in some head movement to help communicate and exaggerate the state of shock he's in. It'll really help your audience connect with your characters, too. In the fire scene, I think it would have helped to put some movement in the fire to make it look more convincing. the static image made it look more like a painting than anything. I liked the motion tween, though.
I don't know what to think about the choice of music. On one hand, it fits the theme really well, but on the other hand, it's not the kind of music one would expect in an anime teaser. It's kind of like hearing Beethoven's "Moonlight Sonata" for a dramatic fight sequence. On one hand, it's really emotional, but on the other hand, well... you get the point.
-Finally, your Japanese is a little off. "Tenshi no you na gakuen" Literally translates to "An academy like an angel." A better English-to-Japanese translation could possibly be "Tenshi no danshi gakuen," but even that is a little iffy. My Japanese isn't the best. ^^;
Overall, it's a pretty neat anime teaser. It has a good premise and it's emotional, but the technicalities could use some work. Other than the things I pointed out, though, I like it. ;D